(1) Field of the Invention
The present invention relates to a steel article coated with a novel pre-treatment coating for receiving an organic coating such as paint, and to a process for applying such a pre-treatment surface coating to steel.
(2) Description of the Prior Art
Steel panels and other components are used extensively in the construction of motor vehicle bodies and other structures. The problem of corrosion of steel by environmental factors is well known, and much work has been carried out to provide steel with coatings to reduce corrosion. Zinc coatings are widely used in the protection of steel strip against corrosion. In the automobile industry the introduction of zinc coatings in conjunction with phosphate and/or chromate treatment processes and multiple paint layers has provided six or more years of protection. The phosphate or chromate treatment is necessary to ensure that the zinc-coated surface is sufficiently corrosion resistant and can be electrocoated with a sufficiently coherent paint layer.
In addition to the barrier protection provided by these coatings the zinc can act in a sacrificial manner to prevent rust formation if the steel is exposed by scratching or stone-chipping to the atmosphere. The zinc-coated steel is also capable of being formed to shape and welded.
In the past 15 years many attempts have been made to improve the corrosion resistance of zinc coatings through alloying of the zinc, for example as disclosed in Japanese Examined Patent (Kokoku) number 50-29821. Electrodeposited Znxe2x80x94Ni alloy has been widely used to protect steel sheet products, with an improvement in corrosion resistance compared to Zn coatings. Typically, over 12 wt % of nickel is incorporated to provide an improved coating. Other attempts at corrosion resistance improvement include the dispersion of inorganic substances in the zinc, for example as disclosed in EP 0 174 019.
It is an object of the present invention to provide a zinc-based pre-treatment coating for a steel substrate, which has improved corrosion resistance. A further object is to provide a zinc-based pre-treatment coating which is suitable for electrocoating without the need for a phosphate or chromate treatment.
According to one aspect of the present invention there is provided a steel article at least a part of a surface of which is plated with a pre-treatment coating layer comprising at least 90% zinc, plus cobalt, at least one trivalent or higher-valent metal, and at least one colloidal inorganic material.
All percentages are given by weight unless otherwise specified.
In a preferred embodiment, the coating comprises:
92 to 99% zinc;
0.5 to 5% cobalt;
0.05 to 1.5% of at least one trivalent or higher-valent metal; and
0.4 to 5% of at least one colloidal inorganic material.
The steel article may be steel strip suitable for use in manufacturing motor vehicle bodies.
It is preferred that the higher-valent metal is chromium or molybdenum. Suitable colloidal inorganic materials include silica, alumina, and ferric oxide. A preferred colloidal inorganic material is silica, notably silica having a particle size range of 5 to 30 nm, preferably 10 to 20 nm. For convenience hereinafter, the invention will be described with reference to preferred embodiments in which the colloidal inorganic material is silica, but it is to be understood that the invention is not limited to these embodiments.
In a preferred embodiment, the coating comprises:
92 to 99% zinc;
0.5 to 5% cobalt;
0.05 to 0.5% chromium; and
0.4 to 5% silica.
In a particularly preferred embodiment, the coating comprises:
93 to 97.9% zinc;
1 to 5% cobalt;
0.1 to 0.2% chromium; and
1 to 3% silica.
Another aspect of the invention provides a steel article at least a part of a surface of which is plated with a corrosion-resistant coating layer consisting essentially of zinc, plus cobalt, chromium, and at least one colloidal inorganic material.
We have surprisingly also found that suitably coated steel substrates can accept paint without the need for pretreatment by phosphates or chromates, allowing painted steel strip to be produced with fewer production steps and reduced cost.
Without in any way limiting the present invention, the following theory is postulated as a possible mechanism for the improved properties obtained by the coatings. We have found that zinc crystal nucleation and growth occurs in a manner that results in each zinc crystal (about 50 nm thick) being encased in a 4 nm thick zinc oxide film. When zinc is co-deposited with cobalt, the deposits exhibit differential distribution of the other elements where the major portion associates with the oxide layer. Here nickel is believed to be acting as a barrier layer which protects each zinc crystal.
The present invention makes use of the fact that zinc oxide is an n-type semiconductor. Zinc atoms in the oxide may be displaced by tri and higher valency ions, for example chromium. This will limit oxide growth and thus enhance its protection of the underlying zinc metal. To enhance the thickness and stability of the nanoscale-thick oxide, one or more colloids are incorporated, for example colloidal silica. We believe that the oxide layer forms from a colloid of zinc oxide as the pH of the plating bath becomes less acid. The additional colloid is present with the ZnO colloidal suspension in the near cathode regions and becomes occluded in the deposit.
The introduction of tri or higher valency elements in the coating is not a simple step as there are limits to the solubility of such elements in the bath. The introduction of silica and the presence of certain levels of divalent cobalt appear to enhance the occlusion of the trivalent element to be absorbed into the zinc oxide or the zinc metal-oxide interface. This means that both the silica, the divalent metal (Co) and the higher-valent metal are not evenly dispersed in the coating; they exist as concentrated layers (about 3 to 10 nm thick) surrounding each zinc crystal. These additions modify the size and shape of the zinc crystals. It is believed that this helps to produce a surface profile into which the paint layer can interlock and form an effective bond.
To form the coating, the steel article is electroplated in an aqueous solution of the appropriate metal ions, containing a dispersion of the colloidal inorganic material.
Accordingly, a further aspect of the invention provides a process for applying a corrosion-resistant coating comprising at least 90% zinc to a steel substrate, the process comprising electroplating the steel substrate in an acidic solution containing:
zinc ions having a concentration in the range 0.2 to 2.5 mol/l;
divalent cobalt ions having a concentration in the range 0.10 to 1.0 mol/l;
ions of one or more trivalent or higher-valent metals having a total concentration in the range 0.004 to 0.05 mol/l; and
a dispersion of a colloidal inorganic material having a concentration in the range 0.02 to 0.2 mol/l.
It is particularly preferred that the plating solution has components in the following concentration ranges:
zinc, 0.5 to 0.8 mol/l;
cobalt, 0.1 to 0.3 mol/l;
trivalent and/or higher valent metal, 0.01 to 0.03 mol/l;
colloidal inorganic material, 0.05 to 0.1 mol/l.
The process may be carried out as a continuous process on, for example, strip steel, or as a batch process.